1. Field of the Invention
The present invention relates to a magneto-optic memory element on which a laser beam is impinged to record, read out or erase information onto or from the magneto-optic memory element.
2. Description of the Priod Art
Recently, a magneto-optic memory element has been developed, which is used in an optical memory disc storing alterable information. The most effective magneto-optic memory element includes a recording medium made of an amorphous alloy thin-film consisting of rare earth metal and transition metal. Such a magneto-optic memory element is generally stable because the recording bit is not affected by the grain boundary, and a large size recording medium can be easily formed.
The enginering group including the present invention has discovered that the recording medium deteriorates when exposed to oxygen or moisture. In order to protect the recording medium from oxygen and moisture, a magneto-optic memory element has been proposed wherein the amorphous alloy thin-film recording layer is sandwiched between a pair of transparent dielectric layers. A basic structure of the improved magneto-optic memory element is disclosed is copending U.S. patent application Ser. No. 489,889, "Magneto-Optic Memory Device", filed on Apr. 29, 1983 by Kenji Ohta, Akira Takahashi, Hiroyuki Katayama, Junji Hirokane and Hideyoshi Yamaoka, and assigned to the same assignee as the present application. The corresponding European Patent Application was published on June 27, 1984 bearing the publication number 0 111 988 A1. The corresponding Canadian Patent Application is Ser. No. 427,088 filed on Apr. 29, 1983.
The magneto-optic memory element disclosed in the above-mentioned U.S. patent application is useful because the transparent dielectric layers function to effectively combine the Kerr effect and the Faraday effect so as to increase the Kerr rotation angle. However, in the magneto-optic memory element disclosed in the aforesaid U.S. patent application, there is a possibility that the amorphous alloy thin-film recording layer may be damaged due to oxidation caused by the SiO or SiO.sub.2 dielectric layer. In order to further improve the magneto-optic memory element, the present inventors have proposed a magneto-optic memory element wherein an amorphous alloy thin-film recording layer is sandwiched between a pair of transparent nitride films. A typical structure of this magneto-optic memory element is disclosed in copending U.S. patent application Ser. No. 648,741, "Magneto-Optic Memory Element", filed on Sept. 10, 1984 by Akira TAKAHASHI, Yoshiteru Murakami, Junji Hirokane, Hiroyuki Katayama, Kenji Ohta and Hideyoshi Yamaoka, and assigned to the same assignee as the present application. The corresponding European Patent Application was filed on Sept. 17, 1984 and assigned Application No. 84306341.3. The corresponding Canadian Patent Application is Ser. No. 462,506 fled on Sept. 5, 1984.
The nitride films effectively protect the amorphous alloy thin-film recording layer from oxygen and moisture at the center portion thereof. However, the periphery of the element includes a bare recording layer. Thus, there is a possibility that the amorphous alloy thin-film recording layer may deteriorate from the periphery thereof in the magneto-optic memory element disclosed in U.S. patent application Ser. No. 648,741. Furthermore, in U.S. patent application Ser. No. 648,741, an Al reflection layer is provided so as to enhance the S/N ratio in the reproduced signal by increasing the photoelectric-magnetic effects (combined Kerr effect and Faraday effect). However, there is a possibility that the turbidity is formed in the Al reflection film during the formation of the Al reflection film. The turbidity reduces the reflection factor.